Again this is a case of someone not reading the rule correctly. No modern MX bikes has a rear chain guard fitted, but they do have a chain guide that does limit the chance of fingers etc going into the rear sprocket. I believe this was why the exception for CMX/DT was made. Older bikes don't have this.
So now any older bikes that haven't got a chain guard close to the sprocket like most bikes from about 1980 on, will need to get one.
That is just my opinion.